This invention relates to a variable attenuator for raster output scanners, and more particularly, to a variable attenuator with a liquid crystal cell and a double-sided polarization plate for electronically controlling dual scanning beam intensity in raster output scanners.
Raster output scanners create or write images in accordance with the image content of an image signal. Typical present day raster output scanners are xerographic based and the images are written on a photoreceptor. The moving photoreceptor, having been previously charged, is exposed line by line by a high intensity beam of electromagnetic radiation, such as a laser, that has been modulated in accordance with input image signal. The modulated beam is focused by suitable optical elements to a point on the photoreceptor and scanned across the photoreceptor by a scanning element such as a rotating multi-faceted polygon. As a result, latent electrostatic images representative of the input image signal are created on the photoreceptor and thereafter developed by the application of a suitable toner thereto. The developed images are then transferred to copy sheets and fixed to provide permanent copies.
Much higher copy output speeds will be needed for future raster output scanners along with better and higher image resolutions. As an answer to this need, scanning systems employing dual scanning beams are contemplated with the intent of minimizing the rotational speed requirements imposed on the scanning element such as a polygon. This would allow the life of the scanning element shaft bearings, a critical and limiting factor to the speed at which the scanning element can safely and reliably operate, to be extended. For dual beam applications, however, a unitary or one piece modulator rather than two separate modulators is considered to be desirable. Unitary modulators consist of a single modulator crystal of suitable size having two separate sets of drive electrodes such that two separate and distinct channels are formed on the same crystal. This reduces cost and optical alignment problems since only a single modulator need be located and fixed in the optical path rather than two. The problem, however, is that for this type of modulator, two very accurately positioned and balanced intensity input beams are essential since the ability to move and adjust a separate modulator for each beam is sacrificed to obtain the advantages offered by the one piece dual modulator design.
Control over the light exposure level at the xerographic photoreceptor is required in all laser printers if acceptable prints and copies are to be produced. Indeed, imaging beam intensity is critical if the proper exposure level for the particular photoreceptor used is to be assured, and if variations in intensity across the scan line and from scan line, and in the laser output power, and in the transmittance, reflectance, and throughput efficiency of the various optical components are to be compensated for. Exposure control takes on added importance and criticality with increased print resolution, half-toning, single-pass highlight color, and other developments where an intensity variation of no more than +/-1 percent is desired.
Where gas lasers are used as the scanning beam source, light intensity is not directly variable at the source. In the past, if intensity control were to be provided, the drive power to the acousto-optic modulator was typically controlled. This allowed the diffraction efficiency of the modulator to be adjusted which in turn controlled the intensity of the scanning beam and provided the exposure levels desired. Uniform exposure across the photoreceptor together with other performance factors requires that the modulator be operated at saturating drive power levels, precluding control over beam intensity by controlling modulator drive power.
Since exposure control through adjustment of the modulator drive power is no longer an available option, other control techniques have been considered and tried but without success. These control techniques include the addition of neutral density filters to the scanner optical system or supporting the entire laser tube assembly for rotation, so as to permit the laser to be turned for optimum polarization with respect to the modulator. Unfortunately, the use of neutral density filters and adjustment of the laser tube assembly can only be implemented manually, greatly limiting their desirability. Furthermore, the use of neutral density filters can induce flare light and cause beam aberrations, while permitting adjustment of the laser tube assembly can result in pointing errors in the laser beam and require subsequent realignment of the optical components following each adjustment.
A twisted nematic liquid crystal can provide controlled rotation of the plane of polarization of the laser beam, such control being in response to beam intensity monitored at the photoreceptor. A liquid crystal twist cell can be disposed between a linear polarizer and a guest/host-type liquid crystal cell.
U.S. Pat. No. 4,920,364 to Andrews et al., assigned to the same assignee as the present application and incorporated herewithin by reference, discloses a variable attenuator for controlling scanning beam intensity having a twisted nematic liquid crystal for rotating the beam polarization and a polarization analyzer to block out the polarization component normal to the analyzer polarization axis, thereby resulting in beam intensity attenuation.
If a liquid crystal cell is used in the variable attenuator, the liquid crystal rotates the polarization of the light transmitted through the crystal. As the voltage applied to the liquid crystal varies, the amount of polarization rotation of the light transmitted through the crystal also varies.
Sheet polarizers have been suggested to remove the varying polarization of the light caused by the liquid crystal. Sheet polarizers transmit the optical polarization component (P-component) which lies along the polarization axis of the material and absorb the optical polarization component (S-component) normal to the polarization axis of the material. However, this absorption of the S-component is not infinite and a portion of the S-component is transmitted with the P-component. Additionally, the absorption of optical energy results in a local temperature increase of the sheet polarizer which then leads to beam shape distortion.
Polarizers can be optical plates coated on a single side with a multi-layer dielectric thin film coating designed such that when tilted close to the Brewster angle, the polarizer plate will transmit over 95 percent of the P-component and reflect over 99 percent of the S-component. Again however, a portion of the S-component of less than 1 percent will be transmitted through the polarizer plate.
Because of this seemingly negligible transmittance of the S-component, if the polarization of the input beam to the polarizer is rotated, the polarization of the output beam from the polarizer will also rotate away from the polarizer axis instead of being exactly along the polarizer axis, which is a main function of a polarizer in the first place, namely output polarization exactly aligned with the polarizer axis. A polarizer is often used to obtain a highly linear polarization in a controlled direction indicated by the polarizer axis.
If the input beam is to be used to create a dual beam by using polarization sensitive optics, still further problems arise. When the output polarization rotates by just a few degrees, large intensity variations are created between two beams generated, for example, in a beam splitting crystal.
However, the physical quantities of the optical beam most effected in the beam splitting crystal by the S-component transmittance are the amplitude of the optical field, i.e. the electric field magnitude and direction of the optical field. Optical intensity is proportional to the square of the electric field amplitude. Therefore, even though a less than one percent (0.01) intensity transmittance of the S-component seems negligible, its amplitude is a significant 0.1.
Thus, as shown in FIG. 1, an incident beam would have an input polarization, E.sub.in, at an angle .theta. with respect to the polarization plate P-component axis. Decomposing along the S and P directions, the beam would have EQU S-comp=E.sub.in sin .theta. [Equation 1] EQU and EQU P-comp=E.sub.in cos .theta. [Equation 2]
If the S-component intensity transmission, T.sub.s, is very small and the P-component intensity transmission, T.sub.p, is large ( 95%). The emerging E fields amplitudes (S-comp.sub.out and P-comp.sub.out), since the amplitude is proportional to the square root of the intensity, will be EQU S-comp.sub.out =.sqroot.T.sub.s E.sub.in sin .theta. [Equation 3] EQU and EQU P-comp.sub.out =.sqroot.T.sub.p E.sub.in cos .theta. [Equation 4]
This square root effect also affects the polarization angle out of the polarizer. Instead of having the output polarization almost entirely in the P direction, it is at an angle .epsilon., where EQU Tan .epsilon.=(S-comp.sub.out /P-comp.sub.out)=.sqroot.(T.sub.s /T.sub.p) tan .theta. [Equation 5]
If the polarization plate has an S rejection ratio of 100:1 for the S-comp, then T.sub.s =0.01, and .sqroot.T.sub.s =0.1.
The light incident to a beam splitting crystal is normally at an input angle, .theta. of 45.degree., to produce dual beams of equal intensity. The incident beam from the polarizer with a T.sub.p of 0.95 and T.sub.s of 0.01, the angle .epsilon. off of the polarizer axis is found to be 5.85.degree.. An angle .epsilon. of 5.85.degree. will cause the two beams produced by the beam splitter to differ in intensity by 44 percent, which is outside the parameters of producing dual beams of equal intensity for a raster output scanner.
Polarization plates are also difficult to manufacture. The angle of incidence has been found to be unpredictable. To obviate this, the polarization plate may be supported in an adjustable cylindrical housing or barrel in order to permit fine adjustment in the orientation of plate to be made.
An expensive solution would be to add a liquid crystal cell to compensate the polarization rotation back. It would basically be an electronic substitute to the barrel rotation. However, it would involve a feedback mechanism similar to the one in U.S. Pat. No. 4,920,364.
It is an object of this invention to provide a variable attenuator for electronically controlling dual scanning beam intensity.
It is another object of this invention to provide a variable attenuator with a single liquid crystal and a double-sided polarization plate for producing dual beams of equal intensity from a beam splitter.